If Rocky is nominated, that would be the only scenario where Johnson might pick Bobby Kennedy as his running mate to secure the youth, Catholic and Northeastern vote. While Kennedy might refuse, he would have to be asked. As for the South: they'd probably vote for Johnson because the Reconstruction folk tales are still alive and well. They get a traitor to his region (LBJ) a liberal Republican (Rocky) and the Southern Antichrist (RFK). So you can imagine security will be very tight in that election.